The present invention relates to a control method and apparatus for a pressure-type furnace for pouring molten metal.
A conventional pressure-type furnace for pouring molten metal is provided with a main body, a reservoir adapted to store the molten metal, an inlet passage through which the molten metal are supplied to the reservoir, and an outlet passage through which the molten metal in the reservoir is supplied to a molten metal pouring chamber. The passages are coupled to lower portions of the reservoir. A gas with a predetermined pressure is supplied to a pressure chamber disposed above the molten metal in the reservoir. The pressure of the gas forces a flow of molten metal through the outlet passage and thereby elevates the level of the metal in the pouring chamber. This level determines the rate at which the metal is poured through an opening in the pouring chamber into a mold.
The pressurized gas is supplied from a pressure supply to the pressure chamber and is controlled by a pressure control device to regulate the level in the pouring chamber and thereby the rate at which the molten metal are poured into the mold. In order to maintain a desired rate of pouring it is necessary to control the value of the gas pressure P in the pressure chamber in accordance with the amount of molten metal in the furnace. The gas pressure should maintain the level of the molten metal in the supply chamber at a constant level. Pressurized air is supplied to the pressure chamber momentarily to increase the pressure and thereby increase the level of the molten metal in the pouring chamber to pour the metal into the mold.
In a pressure-type furnace as described above, when the molten metal stored in the pouring chamber is poured into the mold at the same time that the metal are being replenished, the pressure in the reservoir increases as the amount of the molten metal in the reservoir increases. Consequently, the level of the metal in the pouring chamber increases. This results in variations in the amount of the metal poured in one operation. In order to prevent the variations, a solenoid valve is energized to an on-state to release an amount of air in the pressure chamber when the metal are poured into the mold during replenishment. As a result, excess air in the pressure chamber is exhausted so as to maintain the level of the molten metal in the pouring chamber at the proper level.
In order to accomplish the above-mentioned control operation, it is necessary to detect whether or not the molten metal are being replenished. A variety of methods for detecting this condition of the furnace have been proposed. One conventional detecting method includes the steps of: detecting the weight of molten metal in a furnace by means of a load cell disposed below the main body of the furnace; comparing an output signal of the load cell with an output signal of a potentiometer to obtain a signal representative of the fact that the molten metal are being replenished; and applying the signal indicating the metal are being replenished to a servo motor which drives a booster relay to control the pressure in the pressure chamber.
According to the above-described conventional method, however, a time delay in detecting replenishment may occur due to the presence of the servo motor and the potentiometer. In general, the replenishment of the molten metal is carried out very quickly, as, for example, by the use of a ladle or the like. Accordingly, a problem arises since the gas in the pressure chamber cannot be exhausted quickly enough to eliminate undesired pressure increases. It is thus difficult to control the amount of the metal poured at a desired rate with high accuracy due to the undesired increase in the pressure in the pressure chamber.
Another method of detecting the replenishment of the metal has been proposed in which the difference between the pressure of a pressure control device and the pressure in the pressure chamber is detected by means of a fine pressure difference detector, thereby indicating the replenishment of the metal. In this case, however, as in the abovedescribed method, a problem arises since a time delay in an operation of the detector also occurs, making it difficult evenly to control the pouring of the metal.
In addition, since both of the aforementioned detecting methods utilize complex mechanisms, they have short operating lives and have a relatively large down time when maintenance is required.